In Paper I we have discussed the observed [C ii] deficit, that is the decrease in the [C ii]/FIR intensity with increasing FIR intensity, and related it to a decreased photoelectric heating efficiency in dense PDRs, in addition to the importance of [O i] cooling. Here, we examine the heating efficiency of PDR material in more detail. We estimate the photoelectric heating efficiency by the observed cooling efficiency of the gas, which we approximate by the observed [C ii]/FIR ratio. At intermediate temperatures and densities, the cooling is dominated by cooling through the [C ii] line. At higher temperatures and densities, like those in the Huygens Region, the [O i] 63 \(\mu \) m line becomes important [1]}. However, in the shell surrounding the Extended Orion Nebula, gas cooling is most likely dominated by [C ii], as the temperatures and densities are rather moderate. In M43 and the densest part of NGC 1977 (in OMC3), [O i] emission will contribute significantly [2]}, [3]}.
